Hydraulic bond between casing and formation wall



Dec. 17, 1968 BOUGHTON 3,416,602

HYDRAULIC BOND BETWEEN CASING AND FORMATION WALL Filed June 29, 1965 2 Sheets-Sheet l ouf l |l l II :I

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Lowe Bag/ r BY W HTTOR/VEY United States Patent HYDRAULIC BOND BETWEEN CASING AND FORMATION WALL Lowell D. Boughton, Tulsa, Okla., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed June 29, 1965, Ser. No. 467,895 6 Claims. (Cl. 166-29) ABSTRACT OF THE DISCLOSURE The securement of metal casings in wells by use of a settable cement slurry is facilitated and improved by circulating a mixture of liquid CO and water in the casing during the setting of the cement in place whereby the tendency of the casing to shift or float is lessened and the casing is maintained at a reduced temperature by the mixture which counteracts the heat of hydration of the cement, thus avoiding displacement and damage to the casing due to expansion during cement hydration and the consequent contraction after such heat has been dissipated.

This invention is in the art of securing casing in place and sealing off water in general, and more particularly in the art of cementing casings in boreholes penetrating subterranean formation, including shafts and wellbores wherein a fluid-tight seal is sought to be provided between the casing and the borehole wall.

In most boreholes penetrating subterranean formations it has been found necessary to install a casing (usually of steel, although other metals or concrete may be used) in the borehole to prevent sloughing of the formation wall, to provide a confined conduit for the movement of fluids in the borehole, and to provide a means for inhibiting the intrusion of undesired liquids into the borehole and into adjacent strata. The term casing, as used herein, means any liner or casing for shaft, tunnel, or borehole of any size, including those of concrete as well as those of metal.

The provision of a casing alone is generally not sufficient to inhibit the intrusion of such fluids as water or brine to an extent that permits satisfactory use of such cased boreholes because a space or gap usually exists between the installed casing and the face of the borehole which permits such fluids to flow more-or-less vertically along the casing and become mingled with the fluid sought to be produced. Even when no gap exists immediately after installation of a casing, the pressure of the ground water, usually containing chemically active substances dissolved therein, very often causes the water to begin to seep along the casing into other strata where it causes at least inconvenience and very often seriously interferes with continued use of the borehole unless such seepage is stopped.

The casings are usually secured in place by the use of an hydraulic cement which is intended to maintain the casing in place and to fill the gap between the casing and the borehole wall. However, cement or other bonding material emplaced and allowed to set according to conventional techniques has sometimes provided inadequate protection which has resulted in delays, economic loss, damage equipment, and personal injury due to water seepage past the set cement. For example, in vertical shafts, of suflicient diameter (sometimes up to 6 feet or more) to permit lowering and raising of equipment and personnel, such sepage past the set cement may lead to great damage and injury to such equipment and personnel in subjacent strata since the seepage occasionally reaches a flood stage without adequate warning. For a further example, water or brine flooding of oilor gasproducing strata may completely stop production from a producing well.

Some of the difliculties that continue to exist after a casing has been cemented to the borehold wall are due to the nature of cement itself among which are those due to the expansion of the steel casing during the setting of the cement as a result of 1) the heat of hydration of hydraulic cement during set and (2) the stretching of the casing due to the force of the hydraulic pressure of the aqueous cement slurry and the displacing liquid, e.g. water or mud injected above the cement slurry to force it into position. The expansion of the casing during the setting of the cmeent, after removal of excess cement slurry and the setting of the emplaced cement is followed by contraction of the casing, thereby creating a gap either between the casing and the set cement or between the set cement and the borehole. wall and sometimes gaps are formed in both places.

In a typical cement job using Class A cement slurry, there are approximately 88 British thermal units (B.t.u.) of heat liberated by each pound of cement in 12 hours during the setting thereof. To illustrate the immensity of heat produced on a job employing a 2200-foot casing of the nature illustrated in the annexed drawing, 1480 sacks of cement (94 pounds per sack), 12,240 Btu. would be released in 12 hours. Furthermore, the hydraulic pressure due to water employed as the displacing liquid on 2200-foot casing amounts to 953 pounds per square inch (psi). The combined effect of the hydrostatic pressure and the heat of hydration of the set cement will cause a 20-inch diameter casing of conventional steel to enlarge 0.01485 inch in diameter, during the setting process. This amount of enlargement will be substantially equal to the subsequent shrinkage or contraction of the steel casing after the cement has set, thereby leaving one or more gaps or cracks totaling about 0.01485 inch somewhere between the formation wall and the casing, usually largely immediately adjacent to the casing.

The term settable herein refers to gelation, hydration or the like wherein an aqueous fluid is converted to a hard solid upon standing at subterranean formation conditions.

The invention provides a method of cementing casing in boreholes which obviates to a large extent the problems associated with expansion during emplacing and the setting of the cement slurry and subsequent contraction of the casing.

The invention comprises emplacing an aqueous cement slurry or other bonding sealant material in the annulus between the casing of a borehole and the borehole wall and then circulating a mixture of CO and water in the casing during the setting of the cement whereby the CO and water mixture provides 1) the necessary cooling to counteract the heat of hydration of the setting cement and (2) a low density fluid which is of suflicient density to force and maintain the cement slurry in place and prevent shifting or floating of the casing during the setting period but which is of insuflicient density to produce unwanted strain on the casing. The CO is admixed in the liquid or solid state with the water. The CO changes to a gas shortly after entrance into the borehole. This is best attained by feeding liquid CO and water simultaneously down the wellbore whereupon they mix and shortly thereafter the CO gasfies to provide the water-gaseous CO mixture. The water-CO mixture may be pumped down conventional tubing and back up the annulus between tubing and casing, or in the reverse direction, i.e., down said annulus and back up the tubing.

In larger diameter casings, e.g., those providing shafts extending into subterranean formations for use of equip ment and personnel, wherein tubing may not be employed during use of the borehole, drill casing or other pipe may be used to provide a circulating system for the water-CO mixture, inside of the casing to be cemented. When cementing casing of a size normally used for casing oil wells which employ a tubing to conduct the oil and the fluid, it has been preferred to provide a packer in the annulus near the lower end of the tubing and employ the same tubing and annulus between tubing and casing for the water and CO mixture circulation as are used in production.

In the annexed drawing:

FIGURE 1 shows a borehole representing one of appreciable depth, e.g., one used for oil or gas production, penetrating a subterranean formation having a casing therein which is to be cemented along its entire length below ground level to the Wall of the formation.

FIGURE 2 shows the same borehole as in FIGURE 1 after the aqueous hydraulic cement slurry has been pumped into place.

FIGURE 3 shows the same borehole as in FIGURES 1 and 2 during which awater and CO fluid is being circulated inside the casing while the cement is setting in accordance with the invention.

FIGURE 4 shows a borehole of relatively shallow depth and representative of one of relatively large diameter such as is employed to raise and lower equipment and personnel. It also is provided with a casing. An aqueous hydraulic cement slurry is shown in only the lower part of the annulus between the casing and borehole wall (rather than over the entire length of the casing below ground level). It is provided with a water sealant chemical composition in the annulus above the cement slurry. The chemical composition may or may not be set, at this stage, to a water impenetratable solid.

FIGURE 5 shows the same borehole as that of FIG URE 4 but in a later stage of the cementing process wherein cooling and hydrostatic pressure are provided by a water-CO mixture in accordance with the invention.

Illustrative of the cooling composition for use in the method of the invention is one consisting by weight of between about 60 and 40 parts of water and between about 40 and 60 parts of CO to make 100 parts total weight of composition. A particularly satisfactory proportion of CO and water to employ in the mixture is one of about 50 parts of each by weight, e.g., 45 to 52% of water and a corresponding percent of CO to make 100% by weight.

The proportion of each of water and CO employed is dependent largely upon the density of the fluid desired. It should provide an adequate hydrostatic head to force and/ or hold the casing and cement slurry or other bonding material in place but not such excess weight as to tend to strain or stretch the confining casing. Illustrative of the cooling composition is one consisting of 53% of H 0 and 47% of CO by weight. These proportions of water and CO where the water employed has an initial temperature of about 80 F., result in a temperature of about 70 At this temperature, the water present weights about 4.5 pounds and the CO present about 0.31 pound per gallon of mixture, to give a total density of about 4.81 pounds per gallon.

The temperature of 70 F. is usually considerably cooler than that of the formation and of the casing.

The water-CO mixture may be conveniently prepared by introducing the CO preferably as a liquid, into the water stream as the water is being pumped down the tubing or casing of the wellbore. However, if desired, a premixture may be used by the need for alacrity in injecting the premixture makes this mode of operation much less desirable, generally.

It is recommended (although by no means necessary) during the injection of the CO into the water and the circulation of the resulting mixture inside the casing to be cemented in place, that a pressure in excess of that necessary to force the CO into the water and to circulate the resulting mixture be applied at the surface and therefore transmitted through the circulating system to create a greater pressure at the bottom of the borehole than that produced by the fluid alone. Such additional pressure provides assurance that the system is maintained closed and lessen the likelihood of undesirably low downhole pressures and the possibility of air entering the water- CO mixture. A recommended positive pressure is one between about 25 and 250 pounds per square inch (p.s.i.).

In FIGURE 1 there is shown surface string 1 previously cemented in place to borehole wall 2 of formation 3 by set conventional hydraulic cement 4 which was emplaced according to conventional practice prior to the practice of the invention and is completely set. Casing 6, to be cemented in place according to the invention, is shown extending to a depth just short of the total well depth. It provides annulus 7 between it and borehole wall 2. The lower end of casing 6 is provided with unidirectional fill-up shoe 8 which permits passage of fluid downwardly therethrough but not upwardly. Leading plug 12 is shown approaching the fill-up shoe followed by a precalculated amount of aqueous hydraulic cement slurry 13 and that in turn is followed by trailing plug 15. The leading plug employed is of the type which will rupture and permit free fluid flow therethrough when forceably brought into contact with the fill-up shoe. Mud 16 is shown filling the casing below plug 12 and above plug 15 and the annulus between casing 6 and borehole wall 2 in the lower part of the wellbore and between casing 6 and surface casing 1 in the uper part of the wellbore. There is also shown cap 17 having an opening therein and located at the top of the well for controlled passage of fluids into or out of casing 6. Pipe and valve assembly 18 is shown for use as needed for passage of fluids into or out of annulus 7.

FIGURE 2 shows the same well as that of FIGURE 1 at the progressive stage of the cementing process of the invention, wherein leading plug 12 has been ruptured by having been forced downwardly, by injection of displacing mud 16 shown above trailing plug 15 in casing 6, into contact with fill-up shoe 8. Such contact permits aqueous cement slurry 13 to be forced (by continued ap plication of pressure on the mud above trailing plug 15) around the lower end of casing 6 and thence upwardly into annulus 7, displacing mud from the annulus. By precalculation of the amount of cement slurry needed, the cement slurry employed completely fills annulus 7 as trailing plug 15 reaches fill-up shoe 8, in the position as shown in FIGURE 2.

FIGURE 3 shows the same well as that of FIGURES 1 and 2 at a later stage than FIGURE 2 in the cementing process of the invention wherein tubing 20, of convenient and available size, has been run down in casing 6 to a point just short of plug 15, thereby providing annulus 21 between the tubing and casing. A mixture of water and CO is shown being circulated (by pumping means not illustrated) in a continuous manner down tubing 20 and up annulus 21, thereby displacing mud 16 from the casing, thereby cooling the casing wall in contact with the setting cement in annulus 7 and providing a hydrostatic pressure sufficient to maintain the cement in place and preventing damage to the casing by the inward force created by the cement slurry and mud in the annulus, but yet of insuflicient hydrostatic pressure to cause outward or radial strain on the casing during the setting period of the cement slurry. FIGURE 3 is also shown provided with collar and flow control pipe assembly 19 which permits passage of fluid into tubing 20 and passage of fluids out of annulus 21.

FIGURES 4 and 5 are representative of a borehole of relatively large diameter, of a total depth of about 2300 feet.

FIGURE 4 shows the borehole provided with surface string or casing 1a, which is cemented in place to borehole wall 2a of formation 3:: by conventionally emplaccd cement 4a; casing 6a to be cemented according to the invention, extending to a depth of about 2250 feet, concentrically positioned therein and providing annulus 31 between it and either casing 1a near ground level or to borehole wall 4a therebelow; drill pipe 32 containing mud 16a, extending to a depth of about 2100 feet and positioned in casing 6a providing annulus 33, also filled with mud 16a, between it and casing 6a. Packer 34 is shown in closed position in annulus 33 at the lower end of the drill pipe. Borehole 2a is shown traversing, as it passes downward, a series of aquifers 36, calcite caprock 38, anhydrite rock 40, and terminating in salt stratum 42. Numeral 44 identifies outlet assembly for removal of fluids from annulus 33. FIGURE 4 also shows a low grade economical Pozzolan aqueous cement slurry 53 in annulus 31, extending from ground level to about the 1900 foot level. Directly below the Pozzolan cement, for a vertical distance of about 50 feet, it is aqueous sealant comopsition 54. Below the sealant composition is aqueous cement slurry 13a. These slurries had been positioned by injecting them in successive order down the drill pipe, employing leading plug 12a ahead of the Pozzolan cement and employing separatory plugs 14a and 14b ahead and behind the sealant composition and employing trailing plug a behind aqueous cement slurry 13a. Each of plugs 12a, 14a, and 14b have been ruptured upon having been forceably brought into contact with unidirectional fill-up shoe 8a and are represented in such ruptured state in FIGURE 4. (Plug 15a is also of the ruptura'ble type but has not been yet ruptured).

The same wellbore as that of FIGURE 4 is shown in FIGURE 5, but in a later stage of the cementing process wherein trailing plug 15a has also been ruptured and wherein packer 34 has been opened and a mixture of water and CO is being circulated down drill pipe 32 and up annulus 33 to provide a light-weight cooling medium during the cement setting stage.

FIGURES 4 and 5 illustrate a special application of the invention to cementing off shaft liners and casings by use of both hydraulic cement and a gelable chemical sealant as described in S.N. 371,665, filed June 1, 1964 wherein a selected polymer, e.g. cross-linked polyacrylamide, is dispersed in an aqueous solution of water and a high concentration of at least one selected multivalent (including divalent) metal salt, e.g. a mixture of FeCl and CaCl The sealant, While fluid, is forced into place and maintained therein at reduced temperature, as desired, until the sealant has gelled to a water-tight solid.

Reference will be made to the figures of the drawing in the following exampels which illustrates modes of carrying out the invention.

EXAMPLE 1 An oil well, extending into an oil-bearing stratum, of the type shown in FIGURES 1 to 3 was drilled into a fluid-bearing formation. Surface casing of the nature of that shown as 2 in the drawing was positioned therein and cemented by conventional techniques. A caliper log was run to ascertain the volume of aqueous cement slurry needed to cement in a casing of the nature of that identified as 6 in the drawing whereby the entire annulus between the casing and the borehole wall below the surface string and the annulus between the casing and surface string were filled. Mudidentified by numeral 16 in FIGURES 1 to 3 was circulated down the casing and up the annulus. A leading cement plug similar to that identified as 12 in the drawing was positioned in the casing. A suitable amount of cement slurry prepared by admixing dry hydraulic cement and water in a ratio of about 60 parts cement and 40 parts water was pumped down the casing behind the leading plug. Thereafter trailing plug identified as 15 in the drawing was positioned in the casing and forced downwardly. Mud was pumped down the casing following plug 15 thereby forcing the cement slurry down the casing whereby the leading plug was brought into contact with the fill-up shoe of the type identified by numeral 8 in the drawing. The leading plug 12, upon forceably contacting fill-up shoe 8, was ruptured, the cement thereby being pushed out the lower end of the casing and upwardly in the annulus between the casing and the borehole wall displacing mud therefrom through assembly 18 until the cement slurry had reached the ground level. At that time, since the necessary volume of cement slurry was previously calculated, following plug 15 was at the bottom of the casing. A tubing as represented by numeral 20 in the drawing was then promptly run down inside the casing forming annulus 21. A mixture of water and CO in a weight ratio of about 50 parts of each, was pumped down the tubing and circulated back up the annulus between the tubing and the casing, thereby first displacing mud from the casing and thereafter continuing to cool the setting cement and prevent any damage due to the inward pressure against the casing created by the aqueous cement slurry in the annulus between the casing and the wellbore.

Circulation of the water-CO fluid was continued until the cement had substantially set and was then discontinued. Tubing 2 was then removed. The well was there-. after completed by conventional techniques for producing oil from the oil-bearing stratum. The cement job performed according to the invention is not defective as a result of expansion of the casing during the cementsetting period followed by contraction of the casing leaving cracks in the cement or gaps between the cement and the casing. Seepage of water into the oil along or through cracks in the set cement or between the set cement and the casing is thereby more effectively inhibited.

The method of the invention may be practiced on any cementing job wherein a cooling fluid may be advantageously employed. Among such uses are the cementing of large diameter casing including those extending into underground caverns and mines wherein the casing serves more-or-less as a shaft for raising or lowering personnel and equipment. It has special application to the use of expansive cement in cementing operations as claimed in the Robert C. Martin patent application S.N. 371,755 filed June 1, 1964.

EXAMPLE 2 This example was carried out in a borehole requiring a 30-inch casing to be cemented in place and sealed off against passage of water between the casing and the borehole wall. The wellbore is that represented schematically in FIGURES 4 and 5. Prior to the cementing operation, the Well was drilled, to the required depth; surface casing identified as 2a cemented in position; casing 6a centered in the borehole which contained mud; drill pipe 32 centered concentrically in casing 6a; and packer 34 set in closed position.

A pumpable slurry consisting of cross-linked polyacrylamide in a highly concentrated aqueous solution of FeCl and CaCl was employed as the sealant composition. The polyacrylamide had been prepared by copolymerization at room temperature of acrylamide with about 4600 parts by weight of methylene-bisacrylamide per million parts of acrylamide in an aqueous medium in the presence of a peroxide-type catalyst. The sealant composition had been prepared by admixing a proportion of 4 pounds of the cross-linked polymer so made with enough of a brine (consisting of 38.1% CaCl and 4.7% FeCl by weight in water) to make a gallon of the sealant, as described in application S.N. 371,755, filed June 1, 1964.

The aqueous cement slurries and sealant composition shown in FIGURE 4 were injected down drill pipe 32. Leading plug 12a was positioned ahead of Pozzolan cement 53 and separatory plugs 14a and 14b were employed to separate sealant composition 54 from the Pozzolan cement ahead of it and expansive cement 13a, of the nature of that described in S.N. 371,755, filed June 1, 1964 behind it. Trailing plug 15a followed the expansive cement, being forced down the drill pipe and through fill-up shoe 8a by mud 16a. Drill pipe 32 remained filled with the mud. Plugs 12a, 14a, and 14b and also plug 15a were of the type which ruptured upon contacting fill-up shoe 8a. The injected fluids moved down drill pipe 32 in slug manner. The Pozzolan cement slurry preceding the sealant composition which in turn was followed by the aqueous cement slurry were thereby forced successively into annulus 31. The sealant composition was located in annulus 31 below the aquifer next above the salt stratum and extended vertically downward about 50 feet. The expansive cement slurry extended down annulus 31 from the sealant to the bottom of the well, a distance of about 300 feet. Packer 34 was then opened. Circulation of a fluid mixture consisting of 47% water and 53% CO by weight was then pumped down drill pipe 32 and up annulus 33 in direct contact with the inside of casing 6a and thereby cooling both sealant 54 during its gel period and the aqueous cement slurries 53 and 13a during the setting periods. Circulation of the water-CO mixture was continued until the sealant composition and the cement had set. Circulation was then discontinued. The cement heel, if desired could then be drilled out and the well or shaft be put into use.

Example 2 shows that the method of the invention may be advantageously used to provide desired cooling and adequate but not excessive hydrostatic pressure on casing that is being either cemented by an aqueous hydraulic cement slurry or sealed off by a gellable slurry of a polymer in an aqueous solution or by both.

Although mud of the nature of driling mud was used in the examples to force the aqueous cement slurry and/ or aqueous gellable chemical sealant into place and thereafter the water-CO mixture employed, it is to be understood that the water-CO mixture itself may be employed (where the density of mud is not deemed necessary) to force the aqueous slurries into position.

It is also part of the invention to employ CO intermixed with muds themselves as a cooling medium for cement or other settable or gellable compositions during the geling or setting stage.

Having described my invention, what I claim and desire to protect by Letters Patent is:

1. In the method of bonding a casing of a wellbore penetrating an underground formation to the face of the formation adjacent the casing employing an aqueous fluid composition comprising a water-swellable, water-insoluble, water-gellable polymer in an aqueous salt solution which sets to a solid at formation conditions wherein said aqueous fluid composition is injected down the wellbore and located at the level at which said bonding is desired, the improvement comprising admixing nongaseous CO with water at the wellhead and circulating, in the wellbore down to at least the level at which said fluid is located, the resulting mixture of water and CO during at least a part of the setting period.

2. The method according to claim 1 wherein said salt solution is a substantialy concentrated salt solution of a multivalent metal salt.

3. The method according to claim 2 wherein said salt solution consists essentially of a substantially concentrated solution of a mixture of FeCl and CaCl 4. The method of sealing ofl passageways against the passage of fluids between a casing of a wellbore penetrating a subterranean formation having a higher temperature than the wellbore, and the face of the wellbore wall which comprises providing a tubing in the casing, injecting into the passageway to be sealed an aqueous fluid composition of a water-swellable, water-insoluble polymer in an aqueous salt solution settable to a water-sealing solid at formation conditions, injecting a mixture of water and nongaseous CO into the tubing, a substantial portion thereof converting to gaseous CO therein, and thereafter, during at least a part of the time that said fluid composition is setting, circulating a mixture of water and CO down the tubing and back up the annulus between the tubing and casing to provide an adequate hydrostatic head within the casing, without tending to strain the casing due to too great a radial strain and to provide a cooling medium to inhibit the expansion of the casing, having a tendency to expand when heated, and to contract correspondingly when cooled, such expansion and contraction when allowed to exist tending to leave weakesses and cracks in said composition.

5. The method according to claim 4 wherein said salt solution is a substantially concentrated solution of a multivaent metal salt.

6. The method according to claim 5 wherein said salt solution consists essentially of dissolved FeCl and CaCl.

References Cited UNITED STATES PATENTS 2,772,737 12/1956 Bond 166-42 X 2,811,209 10/1957 Elkins l66-43 X 3,183,971 5/1965 McEver l66-21 3,301,326 1/1967 McNamer 166-42 NILE C. BYERS, ]R., Primary Examiner.

US. Cl. X.R. 

